Machine tools of different types have long been operated under both open and closed loop computer control. In an open loop system, the position of the cutting surface of the machine tools is calibrated and then the tool is operated to advance the cutting tool the prescribed distance or depth to perform the desired operation. Open loop systems are more common in drilling and cutting operations such as performed by punch presses, jig borers, drill presses, lathes and milling machines.
Grinding operations are sometimes performed under closed loop control since the position of the abrasive wheel may not provide an accurate measurement of the part being machined. That is, the grinding operation is controlled by measurements taken directly from the part rather than relying on the position of the grinding wheel itself. For example, the system described in U.S. Pat. No. 3,655,652 uses a linear variable differential transformer to accurately measure the workpiece. The output of the linear variable differential transformer is digitized and used to derive an indication of the amount of material removed from the workpiece, the rate of removal of the material from the workpiece, and the degree of out-of-round of the workpiece. The system of the patent does not develop a signal by means of curve fitting techniques nor does it extrapolate to determine the point in time when the desired amount of material has been removed.
The gear tooth grinding system described in U.S. Pat. No. 3,906,677 provides for modification of the cam control by signals stored in a computer. The system avoids sudden, and therefore, large feed movements by interpolating the command and dividing it into a series of smaller increments. There is no direct measurement from the gear tooth being ground and the system does not involve extrapolation to determine when the grinding operation is complete.
In U.S. Pat. No. 4,027,245, the described control system does not utilize extrapolation nor are direct measurements made from the workpiece to determine the rate of material removal. The grinding wheel position is under direct control of the program which responds to the angular position of the workpiece, but without regard for the dimensional change of the workpiece. A position transducer is utilized only for the purpose of the serve system control and lies outside the computer loop.
U.S. Pat. No. 4,502,125 describes a numerical controller for an angular slide grinding machine which accommodates the change in diameter of the grinding wheel in a dressing operation. The described system does not apply curve fitting techniques to the transducer signal nor does it extrapolate to determine the point in time when the desired amount of material has been removed.
The grinder control system shown in U.S. Pat. No. 4,570,385 is designed to operate in a teach mode, wherein manual control movements are recorded by the computer, and an operate mode in which the previously entered and recorded control movements are reexecuted by the system to duplicate the original movements. The system does not apply curve fitting techniques nor does it extrapolate to determine the point in time when the desired amount of material has been removed.
The system shown in U.S. Pat. No. 4,584,796 is directed to a grinding machine which has a dressing tool fixedly mounted to the bed of the machine and a control system which accommodates both the wear on the grinding wheel caused by the dressing tool and the wear on the dressing tool caused by the dressing operation. The system includes registers which may be manually loaded in the situation where a new grinding wheel is used. The system of the patent does not include means for measuring changes to the workpiece and relies on the measurement of the grinding wheel itself. There is no disclosure of curve fitting or extrapolation to determine when the machining operation has removed the desired amount of material.
The system described in U.S. Pat. No. 4,546,573 utilizes optical detectors in conjunction with reference lines to accurately position the workpiece for a grinding operation. The control system does not apply curve fitting techniques nor does it employ extrapolation to determine when the grinding operation should be terminated.
U.S. Pat. No. 4,233,784 addresses the problem of uneven grinding wheel wear in an apparatus for edge grinding lenses. When lenses are edge ground at the same point of the grinding wheel, the resulting wear causes the formation of a groove in the wheel. To prevent this, a sensor is moved over the surface of the wheel to detect the high point of any irregularity and subsequent grinding takes place at the high point, tending to restore the surface to a flat profile. This system does not utilize curve fitting nor does it employ extrapolation to determine the point in time when the grinding operation should be terminated.
An article in the IBM Technical Disclosure Bulletin, Vol. 1P, No. 1, June 1969, pp 152-55, described a numerical control system which compensates for the dimensional inaccuracy of the cutting tool. The control program provides a linear compensation formula and a circular arc compensation formula. The system does not apply curve fitting techniques to measured values nor does it extrapolate to determine the point in time when the grinding should be terminated.
The control system described in the U.K. Patent No. 2,108,024 provides for additional material to be removed from the workpiece if the stored control program has been completed and the gauge responsive to the dimension of the workpiece indicates that the proper size has not yet been achieved. There is no teaching of curve fitting nor is there any suggestion that the machining operation be terminated in accordance with a value determined by extrapolation from the rate of material removal.
In the control system of French Patent 2,478,515, the grinding machine is controlled to remove a specific volume of material in a unit time. The abstract contains no reference to curve fitting, nor extrapolation as a means for determining when the grinding operation is completed.
The abstract of Russian Patent 1,278,181 describes a system for detecting the point where the cutting tool on a lathe first touches the workpiece. The resulting vibrations are transmitted to a sensor which initiates the automated program for control of the cutting tool. The abstract does not mention the use of curve fitting nor the application of extrapolation techniques to determine the point in time when the turning operation is completed.
While the described turning and grinding systems are in some respects similar to the removal of material by the lapping process, there are certain significant differences which make it difficult to successfully apply the same control techniques. For example, the location of the workpiece between the fixed and movable laps makes it difficult to obtain a direct measurement of the workpiece. Additionally, the nature of the process makes the secondary measurements, those obtained from the lap, very noisy. That is, there is no assurance that any one measurement, or even a series of measurements, is an accurate representation of the workpiece dimension. Since the measurements differ from the actual dimension in random fashion, termination of the lapping operation in response to the first sensing of the desired dimension will virtually ensure an oversize part. If the lapping operation is continued until the average of the sensed dimension is at the desired dimension, there is the likelihood that the lapping operation will have proceeded too long a time and the finished part will be undersized.